Many imaging techniques have been applied to cardiac imaging in order to aid in the diagnosis and treatment of cardiac disease. Such imaging techniques include angiocardiography, isotope imaging, two-dimensional ultrasound, cardiac computed tomography, and magnetic resonance imaging (MRI). Of these techniques, MRI offers numerous advantages. For example, MRI permits high soft-tissue discrimination between blood and myocardium without the need for a contrast medium and can provide functional as well as anatomical information.
Functional cardiac imaging is especially important in the evaluation and treatment of cardiac disease and MRI methods that permit evaluation of cardiac motion are particularly useful. Axel et al., U.S. Pat. No. 5,111,820, describes an MRI method based on spatial modulation of magnetization (SPAMM) in which two sets of orthogonal tagging planes are used to image two-dimensional heart wall motion. Translation, distortion, or other change in appearance of the tagging planes permits assessment of cardiac movement. Unfortunately, the tagged magnetization used to establish the tagging planes begins to decay upon completion of the tagging process so that image signals gradually deteriorate. In addition, any decreases in the magnitude of the tagged magnetization contribute to an untagged magnetization that includes no specimen motion information and represents an increase in image background, reducing tag image contrast.
Image contrast in SPAMM images can be improved using the so-called “complementary” SPAMM (C-SPAMM) method. In this method, a component of magnetization with tagging information is separated from a component of magnetization without tagging information by performing measurements with a positive tagging grid and a negative tagging grid. A difference between the measurement results using the positive and negative tagging grids is obtained. Because positive and negative tagging grids produce magnetizations of opposite signs, image signal portions associated with the two tagged magnetizations are effectively summed. Image signal portions associated with the untagged magnetization do not depend on the sign of the tagging grid and are eliminated or substantially reduced by the difference operation. Such methods are described in, for example, Fischer et al., “Improved Myocardial Tagging Contrast,” Mag. Res. Med. 30:191-200 (1993).
While C-SPAMM does permit improved image contrast, the need for measurements with two tagging grids increases imaging time. In cardiac imaging, a patient must typically hold her breath during measurement, and C-SPAMM requires a doubling of breath-hold time. Many cardiac patients find the necessary SPAMM breath-hold period uncomfortably long and are unable to satisfy the longer C-SPAMM breath-hold time. While more than one breath-hold can be used, any patient movement or different breath depth degrades the quality of cardiac images that are intended to display cardiac motion alone. In addition, untagged magnetization suppression in C-SPAMM MRI is less effective when multiple breath-holds are used. Accordingly, improved MRI methods are needed.